Acoustic wave detection probe and photoacoustic measurement apparatus provided with the same

ABSTRACT

In an acoustic wave detection probe provided with a light guide section that guides measuring light such that the measuring light is outputted toward a subject and an acoustic wave transducer that detects a photoacoustic wave generated in the subject by the projection of the measuring light, the light guide section includes a homogenizer that flat-tops an energy profile of the measuring light entered from the upstream side of the optical system, a light condensing member that condenses the measuring light transmitted through the homogenizer, and a bundle fiber which includes a plurality of optical fibers and is disposed such that the measuring light transmitted through the light condensing member enters from an entrance end of the bundle fiber.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of co-pending U.S. application Ser.No. 14/456,568 filed on Aug. 11, 2014, which is a Continuation of PCTInternational Application No. PCT/JP2013/001182 filed on Feb. 27, 2013,which claims priority under 35 U.S.C. §119 (a) to Japanese PatentApplication No. 2012-043595 filed on Feb. 29, 2012 and Japanese PatentApplication No. 2013-033053 filed on Feb. 22, 2013, the contents ofwhich are hereby expressly incorporated herein by reference in theirentirety.

TECHNICAL FIELD

The present invention relates to a probe to be applied to a measurementtarget to detect an acoustic wave, and a photoacoustic measurementapparatus.

BACKGROUND ART

The photoacoustic spectroscopy is a method in which a pulsed lighthaving a predetermined wavelength (e.g., wavelength range of visiblelight, near infrared light, or mid-infrared light) is projected onto asubject and a photoacoustic wave, which is an elastic wave generated asa result of absorption of the energy of the pulsed light by a specificsubstance in the subject, is detected, thereby quantitatively measuringthe concentration of the specific substance (e.g., Japanese UnexaminedPatent Publication No. 2010-012295). For example, the specific substancein the subject may be glucose, hemoglobin, or the like included in theblood. The technology that detects a photoacoustic wave in the mannerdescribed above and generates a photoacoustic image based on thedetected signal is called photoacoustic imaging (PAI) or photoacoustictomography (PAT).

There has conventionally been a problem described herein below in themeasurements using the photoacoustic spectroscopy described above(photoacoustic measurements). That is, the intensity of the lightprojected onto the subject is significantly attenuated due to absorptionand scattering in the process of traveling through the subject. Theintensity of a photoacoustic wave generated in the subject based on theprojected light is also attenuated due to absorption and scattering inthe process of propagating through the subject. Consequently, it isdifficult to obtain information of a deep portion of the subject by thephotoacoustic measurements. In order to solve this problem, it isconceivable that, for example, the magnitude of generated photoacousticwave is increased by increasing the amount of light energy projectedinto the subject through the use of high energy light.

In the case where high energy (1 mJ or more) light required inphotoacoustic measurements is guided by an optical fiber, it is highlylikely that the end face of the optical fiber is damaged and adurability problem of the optical fiber may possibly occur. Generally,when light is inputted to an optical fiber, the end face of the opticalfiber is placed adjacent to the focal position of the condenser lens sothat the beam diameter of the light fits into the core diameter of theoptical fiber. Here, when the light is condensed by the condenser lens,however, the light is focused too narrowly and the energy is locallyconcentrated, whereby end face damage of the optical fiber may progressfrom the energy concentrated point as the origin.

In the meantime, for example, Japanese Unexamined Patent Publication No.2004-193267 discloses that transmission of high energy light is realizedby the use of a bundle fiber whose entrance end is fusion processed(fused bundle fiber) to efficiently reduce the light energy incident ona unit area.

DISCLOSURE OF THE INVENTION

The use of the method described in Japanese Unexamined PatentPublication No. 2004-193267 in the photoacoustic measurements, however,poses a problem that the homogeneity of the energy profile of the lightoutputted from the fused bundle fiber cannot be ensured. This is becausethe homogeneity of energy profile of light when entering the fusedbundle fiber is not ensured by the method described in JapaneseUnexamined Patent Publication No. 2004-193267. Although, it is describedin paragraph 0017 of Japanese Unexamined Patent Publication No.2004-193267 that the light is directed onto the fused bundle fiber as aspot with a diameter identical to that of the fused bundle fiber, thelight is only focused by a lens against the entrance end of the fusedbundle fiber in Japanese Unexamined Patent Publication No. 2004-193267.In this case, the energy profile of light when entering the fused bundlefiber is thought to have the Gaussian distribution as in normal lightenergy profiles. Then, it is presumed that there is an imbalance in theamount of energy of light traveling through each optical fiber of thebundle fiber.

In Japanese Unexamined Patent Publication No. 2004-193267, all that isrequired is to simply transmit light, so that it is not necessary toensure the homogeneity of the energy profile of exiting light. In thephotoacoustic measurements, however, the homogeneity of the energyprofile of the light actually projected to the subject is required fromthe viewpoint of reconstructing quality photoacoustic signals, inaddition to the transmission of high energy light. For that purpose, itis important to eliminate the imbalance in the amount of energy betweeneach light traveling through each optical fiber of the bundle fiber.

The present invention has been developed in response to theaforementioned requirement, and it is an object of the present inventionto provide an acoustic wave detection probe and photoacousticmeasurement apparatus capable of transmitting high energy light andeliminating imbalance in the amount of energy between each lighttraveling through each of a plurality of optical fibers in photoacousticmeasurements.

In order to solve the aforementioned problem, a photoacoustic wavedetection prove of the present invention is a probe provided with alight guide section that guides measuring light such that the measuringlight is outputted toward a subject and an acoustic wave transducer thatdetects a photoacoustic wave generated in the subject by the projectionof the measuring light, wherein the light guide section includes:

a homogenizer that flat-tops an energy profile of the measuring lightentered the light guide section;

a light condensing member that condenses the measuring light transmittedthrough the homogenizer; and

a bundle fiber which includes a plurality of optical fibers and isdisposed such that the measuring light transmitted through the lightcondensing member enters from an entrance end of the bundle fiber.

That is, in the present invention, the measuring light is passed throughthe homogenizer once to flat-top the energy profile and the beamdiameter of the measuring light when entering the bundle fiber iscontrolled by the light condensing member.

In the probe according to the present invention, it is preferably thatthe homogenizer further diffuses the measuring light. In this case, itis preferable that the light condensing member condenses the measuringlight such that a minimum beam diameter D defined by Formula 1 belowsatisfies Formula 2 below in relation to a diameter d of the bundlefiber, and the bundle fiber is disposed such that the measuring lightenters the bundle fiber with the beam diameter D being 0.8 d to 1.2 d.

$\begin{matrix}{D = {2.5 \cdot f \cdot {\tan\left( \sqrt{\left( \frac{\phi}{2} \right)^{2} + \left( \frac{\theta}{2} \right)^{2}} \right)}}} & {{Formula}\mspace{14mu} 1} \\{{0.8\mspace{14mu} d} \leqq D \leqq {1.2\mspace{14mu} d}} & {{Formula}\mspace{14mu} 2}\end{matrix}$

As used herein, f represents the focal length of the light condensingmember; φ represents the spread angle of the measuring light whenentering the homogenizer; and θ represents the diffusion angle of thehomogenizer. The term “diameter of the bundle fiber” refers to a maximumdistance between circumferences of the cores of farthest optical fibersamong the plurality of optical fibers in the bundle fiber. The term“spread angle” refers to an angle by which the beam diameter of thelaser light spreads in connection with the propagation. The term“diffusion angle” of the homogenizer refers to a design angle ofdiffusion, i.e., an angle by which the beam diameter of the laser lightentered as parallel light and transmitted through the homogenizerspreads in connection with the propagation. The “spread angle” and the“diffusion angle” are expressed in full-width of plane angle. Whenmeasuring these angles, it is preferable that the beam diameters aremeasured at ten points or so within the range of propagation distance inwhich a given beam diameter spreads twice as large as the givendiameter, and the angle is obtained from the slope of change in the beamdiameter.

The term “beam diameter” refers to the diameter of a circle centered onthe beam center (normally, position where the beam intensity is maximum)in which about 86.5% of the energy profile of laser light is included,i.e., the so-called 1/e² diameter. In this case, if it is difficult toobtain the beam center due to an irregular distribution of beamintensity or the like, circles in which energy becomes 86.5% may bedrawn exhaustively near the position presumed to be the beam center andthe diameter of a circle having the minimum area among them may be usedas the beam diameter.

In the probe according to the present invention, it is preferable thatthe light guide section includes a beam expander optical systemimmediately before the entrance side of the homogenizer, the beamexpander optical system having an expansion factor that conforms toangular apertures of the plurality of optical fibers so that themeasuring light is expanded to a beam diameter that conforms to theangular apertures of the plurality of optical fibers.

Further, in the probe according to the present invention, it ispreferable that the entrance end of the bundle fiber is fusionprocessed.

Still further, in the probe according to the present invention, it ispreferable that the outer circumferences of the plurality of opticalfibers are covered with a material having high durability againstoptical energy at the entrance end. In this case, it is preferably thatthe material having high durability against optical energy is silica.

Further, in the probe according to the present invention, it ispreferable that exit ends of optical fibers in each of a plurality ofdivided areas divided in an end face arrangement of the entrance end aredisposed according to a relative magnitude with respect to each dividedarea to which the exit ends belong such that an energy profile when themeasuring light exits from the exit ends of all optical fibers becomeshomogeneous as a whole. In this case, it is preferable that theplurality of divided areas is divided according to the distance from thecenter of the bundle fiber.

Still further, in the probe according to the present invention, it ispreferable that the light guide section includes at least one lightguide plate having a connection surface to which at least some of theexit ends of the plurality of optical fibers are connected and an exitsurface from which the measuring light entered from the connectionsurface exits. In this case, it is preferable that the light guide plateis provided in plurality and disposed across the acoustic wavetransducer.

Further, in the probe according to the present invention, it ispreferable that the homogenizer is a light shaping diffuser in whichsmall lenses are disposed randomly on one side of a substrate.

Still further, it is preferable that the probe according to the presentinvention includes a holding section that integrally holds the lightcondensing member and the bundle fiber. In this case, it is preferablethat the holding section integrally holds the homogenizer as well.

Further, it is preferable that the probe according to the presentinvention includes a holding section that holds the entrance end so asto cover the entrance face of the bundle fiber and has a window sectionat a portion where the measuring light enters. In this case, the windowsection may be formed of an ND filter.

Still further, in the case where the holding section is provided, it ispreferable that the probe according to the present invention includes anaperture member having an aperture that allows the measuring light,which is to enter the bundle fiber, to pass through and an aperturemember which is provided at the entrance end of the bundle fiber andgradually reduces the diameter of the aperture toward the entrance endto a size corresponding to the diameter of the bundle fiber.Alternatively, it is preferable that the holding section includestherein a light guide member formed of a cap member and a ring shapedchip made of a material resistant to optical energy and fitted aroundthe cap member or a light guide member having an aperture stop and arelay lens system. Still further, it is preferable that the holdingsection includes a connector structure removably attachable to amountingsection of an equipment housing which includes a light source.

A photoacoustic measurement apparatus of the present invention includes:

the probe described above; and

a signal processing means that processes a photoacoustic signal of thephotoacoustic wave detected by the acoustic wave transducer.

It is preferable that the photoacoustic measurement apparatus accordingto the present invention includes a light source that outputs themeasuring light, an equipment housing having a mounting sectionoptically connected to the light source and holding the homogenizer, anda holding section that integrally holds the light condensing member andthe bundle fiber, in which the mounting section and the holding sectionhave connector structures removably attachable to each other.Alternatively, it is preferable that the photoacoustic measurementapparatus according to the present invention includes a light sourcethat outputs the measuring light, an equipment housing having amountingsection optically connected to the light source and holding thehomogenizer and the light condensing member, and a holding section thatholds the entrance end so as to cover the entrance face of the bundlefiber and has a window section at a portion where the measuring lightenters, in which the mounting section and the holding section haveconnector structures removably attachable to each other.

Further, in the photoacoustic measurement apparatus according to thepresent invention, it is preferable that the signal processing meansincludes an acoustic image generation means that generates aphotoacoustic image based on the photoacoustic signal. In this case, itis preferable that the acoustic wave transducer detects a reflectedacoustic wave of a transmitted acoustic wave to the subject, and theacoustic image generation means generates a reflected acoustic waveimage based on a signal of the reflected acoustic wave.

The acoustic wave detection probe and the photoacoustic measurementapparatus according to the present invention are characterized in thatthe light guide section includes a homogenizer that flat-tops an energyprofile of the measuring light entered therein, a light condensingmember that condenses the measuring light transmitted through thehomogenizer, and a bundle fiber which includes a plurality of opticalfibers and is disposed such that the measuring light transmitted throughthe light condensing member enters from an entrance end of the bundlefiber. This allows the flat-topped laser light to dividedly enter eachoptical fiber in the bundle fiber and damage to the end face of thebundle fiber due to local energy exceeding damage threshold energy to beprevented to a large extent. As a result of this, it is possible totransmit high energy light and eliminate imbalance in the amount ofenergy of light traveling through each of a plurality of optical fibersin photoacoustic measurements.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-sectional view, illustrating an exampleconfiguration of a light guide section of a probe of a first embodiment.

FIG. 2 is a schematic cross-section view, illustrating an arrangement ofan acoustic wave transducer array and an optical fiber of the probe ofthe first embodiment.

FIG. 3 shows schematic cross-sectional views, illustrating a pluralityof example configurations of homogenizer.

FIG. 4 shows schematic cross-sectional views, illustrating other exampleconfigurations of the light guide section of the probe.

FIG. 5 is a schematic view, illustrating an end face arrangement of theentrance end of a fusion processed bundle fiber.

FIG. 6, a shows an energy profile of laser light flat-topped first by ahomogenizer and then condensed by a lens, and b shows an energy profileof laser light only condensed by a lens without using a homogenizer.

FIG. 7 is a graph illustrating a relationship between the opticalproperty of a light shaping diffuser and light condensing member and theminimum beam diameter.

FIG. 8 is a graph illustrating, in the case where laser beam whose angleformed between the traveling direction thereof and the optical axis of alight condensing member has distribution is condensed by the lightcondensing member, a correlation between the diameter of condensingrange and the minimum beam diameter.

FIG. 9 is a schematic view of a light guide section which includes abeam expander, illustrating the configuration thereof.

FIG. 10 is a schematic view illustrating light guide plate configurationexamples.

FIG. 11 is a schematic cross-section view illustrating an arrangement ofan acoustic wave transducer and an optical fiber of the probe of thesecond embodiment.

FIG. 12 illustrates a setting method of divided area divided in an endface arrangement of the entrance end of an optical fiber.

FIG. 13, a is a schematic view illustrating a design change in thearrangement of optical fiber in the probe of the first embodiment, and bis a schematic view illustrating a design change in the arrangement ofoptical fiber in the probe of the second embodiment.

FIG. 14, a is a schematic view illustrating a configuration of amounting section of an equipment housing which includes a light source,and a holding section and b is a schematic view illustrating that theholding section is mounted in the mounting section shown in a of FIG.14.

FIG. 15, a is a schematic view illustrating another configuration of amounting section and a holding section of an equipment housing whichincludes a light source, and b is a schematic view illustrating that theholding section is mounted in the mounting section shown in a of FIG.15.

FIG. 16 is a schematic view illustrating another configuration of amounting section and holding section of an equipment housing whichincludes a light source.

FIG. 17 is a schematic view illustrating a configuration in which anopening member is provided in the holding section.

FIG. 18A is a schematic view illustrating a configuration in which a capmember (silica rod) is provided at the entrance end of the bundle fiber.

FIG. 18B is a schematic view illustrating a configuration in which a capmember (air gap optical fiber) is provided at the entrance end of thebundle fiber.

FIG. 19 is a schematic view illustrating the relationship between theenergy profile of input beam and the cap member.

FIG. 20 is a schematic view illustrating a configuration in which arelay lens system is provided at the entrance end of the bundle fiber.

FIG. 21 is a schematic view illustrating a first embodiment of aphotoacoustic image generation apparatus, as a photoacoustic measurementapparatus.

FIG. 22 is a schematic view illustrating a second embodiment of aphotoacoustic image generation apparatus, as a photoacoustic measurementapparatus.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described withreference to the accompanying drawings, but it should be appreciatedthat the present invention is not limited to these embodiments. Notethat each component in the drawings is not necessarily drawn to scale inorder to facilitate visual recognition.

[First Embodiment of Acoustic Wave Detection Probe]

A first embodiment of acoustic wave detection probe will be describedfirst. FIG. 1 is a schematic cross-sectional view, illustrating anexample configuration of a light guide section of the probe according tothe present embodiment. FIG. 2 is a schematic cross-section view,illustrating an arrangement of an acoustic wave transducer array and anoptical fiber of the probe of the present embodiment.

As illustrated in FIGS. 1 and 2, the probe 11 according to the presentembodiment includes a light guide section 44 which is formed of ahomogenizer 40, a light condensing member 41, and a fusion processedbundle fiber 42, an acoustic wave transducer array 20, and a housing 11a that holds an exit end E2 of the bundle fiber 42 and the acoustic wavetransducer array 20. In the present embodiment, the probe 11 is usedthrough optical connection to a laser unit 13 such that laser light Loutputted from the laser unit 13 enters the homogenizer 40. The laserlight L incident on the homogenizer 40 enters an entrance end E1 of thebundle fiber 42 via a light condensing member 41. Thereafter, the laserlight L guided by the bundle fiber 42 exits from the exit end E2 of eachoptical fiber 42 a in the bundle fiber 42 and is projected onto asubject M as measuring light. Note that the measuring light is notlimited to laser light.

<Housing>

The housing 11 a also functions as a holding member for the operator ofthe probe 11 to hold the probe 11. Although, the housing 11 a has ahand-held shape in the present embodiment, the housing 11 a of thepresent invention is not limited to this.

<Homogenizer>

In the present embodiment, the homogenizer 40 is an optical element thatflat-tops the energy profile (energy distribution) of the laser light Lentered from the upstream side of the optical system and diffuses thelaser light L. The flat-topped laser light L is guided to the lightcondensing member 41, and enters the entrance end E1 with theflat-topped energy profile being maintained. To “flat-top” the energyprofile is, in other words, to shape the laser light entered thehomogenizer 40 so as to have a flat top energy profile near the center.The term “flat-top” as used herein refers to, when a concentric circlewith a diameter of 80% of the beam diameter is taken in the energyprofile of the laser light exiting from the homogenizer and the standarddeviation is obtained with respect to the energy of each point withinthe concentric circle, the state in which the standard deviation is 25%or less of the average energy within the concentric circle. Generally,the homogenizer is formed such that the light is completely flat-topped(i.e., the standard deviation is nearly equal to 0) at infinity. In thepresent invention, the energy profile of the measuring light whenentering the entrance end of the bundle fiber, however, is notnecessarily in the state of completely flat-topped, and an energyprofile which is in a flat-topped state to the extent of the rangedescribed above is sufficient. The flat-topped energy profile of thelaser light L may prevent the light intensity from being locallyintensified and damage in the bundle fiber 42 is also inhibited.Further, the imbalance between the energy of light enters each opticalfiber is inhibited.

Further, the homogenizer 40 of the present embodiment also functions todiffuse the laser light L outputted from the laser unit 13 to increasethe beam diameter of the laser light L, that is, to spread thedistribution of propagation angles of light beams included in the laserlight L. This causes the emission surface of the homogenizer 40 to serveas the secondary light source of the laser light L so that, when thelaser light is condensed by the light condensing member 41, the laserlight L is prevented from being focused too narrowly. The diffusionangle of the homogenizer 40 is preferably 0.2 to 5.0° and morepreferably 0.4 to 3.0°. The reason for this is high transmissionefficiency. The diffusion function of the homogenizer is not essential.

The distance between the homogenizer 40 and the light condensing member41 is controlled appropriately such that the laser light L transmittedthrough the homogenizer 40 is efficiently coupled to the lightcondensing member 41. At this time, the homogenizer 40 is preferablydisposed on the upstream side of the optical system with respect to thelight condensing member 41 and within a range three times the focallength of the light condensing member 41 from the center thereof.

The homogenizer 40 may be formed of a single optical element or may beformed of a combination of a plurality of optical elements. In the casewhere the homogenizer 40 is formed of a single optical element, forexample, n Shaper available from AdlOptica may be used as thehomogenizer 40. As for the homogenizer having a diffusion function, forexample, a light shaping diffuser 53 having small concave lensesdisposed randomly on one side 53 s (FIG. 4) is preferably used. As forsuch light shaping diffusers, for example, Engineered Diffusers (ModelNumber: EDC-2.0-A, Diffusion Angle: 2.0°) available from RPC Photonicsmay be used. The use of such elements allows the energy profile and theshape of the laser light L to be changed almost arbitrarily. In thisway, if the homogenizer 40 is formed of a single optical element, thelight guide section 44 may be formed of a simple structure.

In the meantime, in the case where the homogenizer 40 is formed of aplurality of optical elements, for example, the following configurationmay be cited. FIG. 3 shows schematic cross-sectional views, illustratingexample configurations of the homogenizer 40. The homogenizer 40 may beformed, for example, by arranging a microlens array A 45, a microlensarray B 46, a planar-convex lens 47, and a variable beam expander 48 inthe manner shown in a of FIG. 3. Further, the homogenizer 40 may also beformed by appropriately combining a holographic diffusion plate 49, alight focusing planar-convex lens 50, and a light pipe 51, as shown in bof FIG. 3. Still further, the homogenizer 40 may also be formed of aflat top laser beam shaper 52 which includes, for example, an asphericallens for correcting beam energy profile, as shown in c of FIG. 3.

The homogenizer 40 may be structured so as to be held integrally withthe light condensing member 41 and the bundle fiber 42 by a holdingsection 60 a, as shown in a of FIG. 4. This may eliminate the need toadjust the positional relationship between the homogenizer 40 and thelight condensing member 41, whereby the optical system may be downsized.

<Light Condensing Member>

The light condensing member 41 guides the laser light L transmittedthrough the homogenizer 40 to the entrance end of the bundle fiber 42,and may be formed of a condenser lens, a mirror, or a combinationthereof. For example, in the present embodiment, the light condensingmember 41 is a light condensing system formed of a single condenserlens. The focal length of the light condensing member 41 (distancebetween the principal point and the focal point on the side of thebundle fiber 42) is preferably 10 to 100 mm and more preferably 15 to 50mm. The reason for this is that it allows downsizing of the opticalsystem and conforms to numerical apertures NA (0.22 at maximum) ofcommon optical fibers in which the core is formed of silica and thecladding is formed of fluorine-doped silica. Further, the lightcondensing member 41 may be a coupled lens system. If the lightcondensing member 41 is a coupled system lens, the focal length of thelight condensing lens 41 refers to the combined focal length of thecoupled system lens. The light condensing member 41 may be formed so asto be held integrally with the homogenizer 40 and the bundle fiber 42 bythe holding section 60 a, as shown in a of FIG. 4 or held integrallywith only the bundle fiber 42 by the holding section 60 a, as shown in bof FIG. 4.

<Bundle Fiber>

The bundle fiber 42 guides the laser light L condensed by the lightcondensing member 41 (i.e., transmitted through the light condensingmember 41) near the acoustic wave transducer array 20. Note that theother light guide member may be provided between the light condensingmember 41 and the bundle fiber 42. The bundle fiber 42 includes aplurality of optical fibers 42 a, each having a core and a cladding, acovering member 42 c, such as a ferrule, a sheath, and the like, and afilling member 42 b filling between the outer circumferences of theplurality of optical fibers 42 a and the covering member 42 c, asillustrated, for example, in FIG. 5. The core diameter of each opticalfiber 42 a in the bundle fiber 42 is preferably 20 to 300 μm and morepreferably 50 to 200 μm. There is not any specific restriction on theoptical fiber 42 a in the bundle fiber 42 but the optical fiber 42 a ispreferably a silica fiber.

Further, in the present embodiment, fusion processing is performed on atleast the entrance end of the bundle fiber 42. The fusion processing isa bundling technology in which, when packing bear optical fibers into abundle, the processing is performed not with an adhesive but heat andpressure. In a fusion processed bundle fiber, the claddings are fusedtogether and the optical fibers are bundled in a hexagonal honeycombshape and an extra gap between optical fibers is eliminated incomparison with the bundling with an adhesive. Therefore, this mayprovide an advantageous effect that the area occupied by the cores perunit area is increased. Also it may provide an advantageous effect thata material susceptible to optical energy is hidden so that thedurability against optical energy is improved. From the viewpoint offurther improving the durability against optical energy of the bundlefiber 42, the filling member 42 b is preferably formed of a materialhaving high durability against optical energy. Such a material may be aglass material, such as silica and the like. Such a bundle fiber may beproduced by inserting a plurality of optical fibers into, for example, acylindrical member of silica or the like, performing fusion processingon the cylindrical member, as well as the optical fibers, and thereaftercovering the circumference with a covering member.

For example, the bundle fiber 42 is positioned such that the entranceend E1 thereof is located at the focal point of the light condensingmember 41. A configuration may be adopted in which a bundle fiberpositioning section (not shown in the drawing) that moves the bundlefiber in the directions of the optical axis thereof may be provided inorder to allow a fine adjustment of the position of the bundle fiber 42.This allows the positional adjustment to be made near the focal pointwithin the range in which the flat top state is not impaired and alsoallows the beam diameter when entering the entrance end E1 to be finelyadjusted.

Further, if the homogenizer 40, the light condensing member 41, and thebundle fiber 42 are held integrally by the holding section 60 a, asshown in a of FIG. 4, or if the light condensing member 41 and thebundle fiber 42 are held integrally by the holding section 60 a, asshown in b of FIG. 4, the bundle fiber 42 is preferably fixed to theholding section 60 a or 60 b by a structure that allows the bundle fiber42 to be removed from or attached to the holding section 60 a or 60 b,such as a screw structure, so that the entrance end E1 of the bundlefiber 42 is fixed easily at the focal point of the light condensingmember 41. In FIG. 4, complementary screw structures are provided in thejointing section 100 a of the hold section 60 a or 60 b on the side ofthe light condensing lens 41 and the jointing section 100 b on the sideof the bundle fiber 42, whereby the bundle fiber 42 is removably fixedto the holding section 60 a or 60 b. If the bundle fiber is fixed to theholding section 60 a or 60 b by, for example, the screw structures inthe manner described above, the bundle fiber positioning section is notrequired and the optical system may be downsized. Further, the bundlefiber 42 may be replaced easily by simply unscrewing from the holdingsection 60 a or 60 b, the position readjustment between the lightcondensing member 41 and the bundle fiber 42 is unnecessary whenreplacing a damaged bundle fiber and the maintenability is improved. Inorder to integrate the light condensing member 41 and the bundle fiber42 via the screw sections with the positional relationship being fixed,for example, the aspherical lens fiber collimator package (Model Number:F280SMA-A or F280SMA-B, Focal Length: 18.4 mm) available from Thorlabs,Inc. or the like may be used. Further, as the aspherical lens fibercollimator package series of Thorlabs, Inc. provides the products havingfocal lengths ranging from about 4 mm to 18.4 mm, it is possible to makean appropriate selection according to the purpose.

In order to improve homogeneity of the energy profile of the lightprojected onto the subject, exit ends E2 of a plurality of opticalfibers 42 a are disposed substantially evenly around the acoustic wavetransducer array 20 on the exit side of the bundle fiber 42 according tothe present embodiment.

<Acoustic Wave Transducer Array>

The acoustic wave transducer array 20 includes a plurality of acousticwave transducers (or acoustic wave detection elements) disposedone-dimensionally or two-dimensionally and converts an acoustic wavesignal to an electrical signal. The acoustic wave transducer is apiezoelectric element made of, for example, piezoelectric ceramics,piezoelectric single crystal, or a polymer film, such as apolyvinylidene fluoride (PVDF) film or the like. The term “acousticwave” as used herein refers to include ultrasonic wave and photoacousticwave. The term “ultrasonic wave” as used herein refers to an elasticwave generated in the subject by the vibration of the acoustic wavetransducer array and a reflection wave thereof, and the term“photoacoustic wave” as used herein refers to an elastic wave generatedin the subject by the photoacoustic effect due to the projection ofmeasuring light. The acoustic wave transducer array 20 preferablyincludes acoustic elements, such as an acoustic matching layer, anacoustic lens, a backing member, and the like, in order to detectaccurate acoustic wave signals.

<Control of Beam Diameter>

In the case where the homogenizer 40 has the diffusion function, thelight condensing member 41 condenses laser light L (measuring light)such that a minimum beam diameter D (i.e., beam diameter on the focalplane) of the laser light L defined by Formula 3 below satisfies Formula4 below in relation to a diameter d of the bundle fiber 42, and thebundle fiber 42 is preferably disposed such that the laser light Lenters the bundle fiber 42 with the beam diameter D being 0.8 d to 1.2d.

$\begin{matrix}{D = {2.5 \cdot f \cdot {\tan\left( \sqrt{\left( \frac{\phi}{2} \right)^{2} + \left( \frac{\theta}{2} \right)^{2}} \right)}}} & {{Formula}\mspace{14mu} 3} \\{{0.8\mspace{14mu} d} \leqq D \leqq {1.2\mspace{14mu} d}} & {{Formula}\mspace{14mu} 4}\end{matrix}$

The reason why the minimum beam diameter D is set to 0.8 d or more is toinhibit damage to the entrance end E1 (core damage mode) of the bundlefiber 42 caused by energy concentration due to the focused beamdiameter, and more specifically it is as follows.

An energy profile of the laser light L at the focal plane which isflat-topped first by the homogenizer 40 and then condensed by a lens isshown in a of FIG. 6. An energy profile of laser light only condensed bya lens without using the homogenizer 40 is shown in b of FIG. 6. FromFIG. 6 it is known that the ratio of the full width at half maximum W1to the minimum beam diameter D1 of the laser light in a of FIG. 6 islarge in comparison with the ratio of the full width at half maximum W2to the minimum beam diameter D2 of the laser light in b of FIG. 6.Generally, as the spread angle φ of the laser light L when outputtedfrom the laser unit is small (about 0.15° at most), the condensed laserlight L is focused to a small size at the entrance end E1 of the bundlefiber 42. As a result, the energy of the laser light L is concentratedat the entrance end of the bundle fiber 42 and damage occurs in theentrance end E1 of the bundle fiber 42.

Consequently, in the present embodiment, the beam diameter of the laserlight L at the focal position is controlled by diffusing the laser lightL once by the homogenizer 40. FIG. 7 is a graph illustrating therelationship between the optical property of light shaping diffuser(Engineered Diffusers) and light condensing member and minimum beamdiameter. The horizontal axis of the graph represents the diffusionangle (deg.) of the light shaping diffuser and the vertical axisrepresents the size of the minimum beam diameter (μm). The circularplots in the graph represent data when the focal length of the lightcondensing member is 100 mm, the square plots represent data when thefocal length of the light condensing member is 50 mm, and the triangularplots represent data when the focal length of the light condensingmember is 25 mm. It is known from FIG. 7 that the minimum beam diametermay be controlled by controlling the optical property of the homogenizerand the light condensing member.

Such beam diameter control method uses the principle that, when parallellight traveling in a direction that forms an angle α with the opticalaxis of a light condensing member is incident on the light condensingmember having a focal length f, the position of the condensing pointwhere the parallel light is condensed is displaced from the position ofthe focal point of the light condensing member and the distance betweenthe condensing point and the focal point may be approximated by f·tan α.

If the angle formed by the traveling direction of a laser beam incidenton a light condensing member and the optical axis of the lightcondensing member has a distribution, therefore, the laser beam iscondensed at a position corresponding to each angle, so that thecondensing range (including peripheral portion) of the entire laserlight in which each condensing point is lapped is increased. Forexample, if a homogenizer having a diffusion function is disposed on theupstream side of a light condensing member, the angle distribution ofthe laser beam which was about φ/2 or less before entering thehomogenizer is extended to within about √((φ/2)²+(θ/2)²) in terms ofhalf angle, so that the entire condensing range of the entire laserlight condensed thereafter by the light condensing member is furtherextended corresponding to this.

Further, considering that the 1/e² diameter of the laser light is set tothe beam diameter in the condensing range, it is presumed that thediameter of the condensing range 2f·tan(√((φ/2)²+(θ/2)²)) and theminimum beam diameter D has a certain correlation with each other.

FIG. 8 is a graph illustrating, in the case where laser beam whose angleformed between the traveling direction thereof and the optical axis of alight condensing member has distribution is condensed by the lightcondensing member, the correlation between the diameter of thecondensing range 2f·tan(√((φ/2)²+(θ/2)²)) and the experimentallyobtained actual minimum beam diameter D. More specifically, the graphshows results of experiment in which laser light having a wavelength of532 nm and a pulse width of 3.5 ns with a beam diameter of 3.5 min and aspread angle φ of 0.13° before being inputted to a homogenizer having agive diffusion angle θ, is inputted to the homogenizer, then the laserlight is condensed by a light condensing lens having a given focallength f, and the condensing range is measured with a beam profiler(LaserCam-HR available from Coherent Inc.). The same beam profiler wasused when the diffusion angle of the homogenizer was obtained. The fivecircular plots in the graph are the results of measurement with anoptical system of a combination of a light condensing lens having afocal length f of 100 mm and a homogenizer, and diffusion angles θ ofthe homogenizer are 0.25, 0.50, 1.02, 2.05, and 3.15° from the leftbottom plot. The five rectangular plots in the graph are the results ofmeasurement with an optical system of a combination of a lightcondensing lens having a focal length f of 50 mm and a homogenizer, anddiffusion angles θ of the homogenizer are 0.25, 0.50, 1.02, 2.05, and3.15° from the left bottom plot. The five triangular plots in the graphare the results of measurement with an optical system of a combinationof a light condensing lens having a focal length f of 25 mm and ahomogenizer, and diffusion angles θ of the homogenizer are 0.25, 0.50,1.02, 2.05, and 3.15° from the left bottom plot.

It is known from FIG. 8 that the minimum beam diameter D is in therelationship of linear function with the diameter of the condensingrange. The slope of the linear function in the graph is about 1.25.Thus, the minimum beam diameter D is given by Formula 3 above.

That is, for a given laser light, it can be said that any minimum beamdiameter D may be formed by appropriately setting the focal length f andthe diffusion angle θ, not limited to the focal lengths and diffusionangles used in the experiment. Then, the wider the beam diameter, themore the energy density may be reduced.

In the present embodiment, it is possible to guide the high energy laserlight L by the bundle fiber 42 without exceeding the damage thresholdenergy density of the entrance end of the bundle fiber 42 by controllingthe minimum beam diameter D of the laser light L through the use of therelationship between the focal length of the light condensing member andthe diffusion angle of the homogenizer.

In relation to the diameter d of the bundle fiber 42, the reason why theminimum beam diameter D is set to 1.2 d or less is to inhibit damage toa member surrounding the entrance end E1 of the bundle fiber 42 byabsorbing the energy of the laser light L if the minimum beam diameter Dis increased and to prevent emissions, such as dust, gas, and the like,from being released from the damaged region. Such emissions induce thedestruction of the cores by adhering to the end face of the bundle fiber42 and can be a cause of the problem that the energy transmission ishindered (ambient damage mode). That is, the reason why the minimum beamdiameter D is set to 1.2 d or less is to inhibit the generation of theambient damage mode described above. The member surrounding the bundlefiber refers to, for example, the filling material 42 b made of resin,the covering member 42 c, such as the metal ferrule covering the outercircumference of the filling member 42 b, and the like.

The reason why the minimum beam diameter D exceeds d is that the portionin the range exceeding d is a peripheral portion (on the remote sidefrom the optical axis) of the beam and has relatively weak lightintensity, and therefore the ambient damage mode is unlikely to occureven the minimum beam diameter exceeds somewhat the diameter of thebundle fiber. The preferable range of the minimum beam diameter D is 0.8d to 1.0 d. The reason why the bundle fiber 42 is disposed such that thelaser light L enters the bundle fiber 42 with the beam diameter D being0.8 d to 1.2 d is to efficiently input the laser light L condensedaccording to the diameter d of the bundle fiber 42 to the entrance endE1 of the bundle fiber 42.

As described above, in the acoustic wave detection probe according tothe present embodiment, light (measuring light) is passed through thehomogenizer once to flat-top the energy profile, and then the beamdiameter of the laser light when entering the bundle fiber is controlledby the light condensing member. This allows the flat-topped laser lightto dividedly enter each optical fiber in the bundle fiber and damage tothe end face of the bundle fiber due to local energy exceeding thedamage threshold energy density to be prevented to a large extent. Theprevention of local damage leads to that more amount of energy may beinjected as a whole from the viewpoint of energy transmission, andindicates that the energy is distributed appropriately to each opticalfiber from the viewpoint of imbalance in the amount of optical energy.As a result of this, it is possible to transmit high energy light andeliminate imbalance in the amount of energy of light traveling througheach of a plurality of optical fibers in photoacoustic measurements.

[Second Embodiment of Acoustic Wave Detection Probe]

Next, a second embodiment of the acoustic wave detection probe will bedescribed. The probe of the present embodiment differs from the firstembodiment in that the light guide section includes a beam expanderoptical system on the upstream side of the bundle fiber 42. Therefore,components identical to those of the first embodiment will not beelaborated upon further here unless otherwise specifically required.

FIG. 9 is a schematic view of the light guide section which include abeam expander, illustrating the configuration thereof.

The probe 11 according to the present embodiment includes a light guidesection 44 which is formed of a beam expander 55, the homogenizer 40,the light condensing member 41, and the fusion processed bundle fiber42, the acoustic wave transducer array, and the housing that holds theexit end of the bundle fiber 42 and the acoustic wave transducer array.In the present embodiment, the probe 11 is used through opticalconnection to the laser unit 13 such that laser light L outputted fromthe laser unit 13 enters the beam expander 55. The laser light Lincident on the beam expander 55 enters the entrance end E1 of thebundle fiber 42 via the homogenizer 40 and the light condensing member41. Thereafter, the laser light L guided by the bundle fiber 42 exitsfrom the exit ends E2 of a plurality of optical fibers 42 a in thebundle fiber 42 and is projected onto a subject M as measuring light.

The housing, homogenizer 40, the light condensing member 41, and theacoustic wave transducer array are identical to those of the firstembodiment.

<Beam Expander Optical System>

The beam expander optical system 55 expands the measuring light to abeam diameter that conforms to the angular apertures of a plurality ofoptical fibers 42 a in the bundle fiber 42 and further to an optimumbeam diameter with respect to the angular apertures, as illustrated, forexample, in FIG. 9. The term “beam diameter that conforms to the angularapertures of optical fibers” refers to a beam diameter in which, whenlight is condensed at the entrance end of the bundle fiber via thehomogenizer and the light condensing member, the convergence angle ofthe light becomes close the angular apertures of the optical fibers. Theterm “optimum beam diameter with respect to the angular apertures of theoptical fibers” refers to a beam diameter in which the convergence angleof the light becomes substantially equal to the angular apertures of theoptical fibers at that time. The beam expander optical system 55 isdisposed on the upstream side of the light condensing member 41 (i.e.,on the light source side). The expansion factor of the beam expanderoptical system 55 is adjusted to the angular apertures of the pluralityof optical fibers 42 a so that the laser light L may be inputted to theentrance end E1 of the bundle fiber 42 with a wider spread angle withinthe range that does not exceed the numerical apertures of the opticalfibers 42 a. For example, the numerical aperture of a silica fiberhaving a general core/cladding structure is 0.20 to 22 and the angularaperture is 11.4 to 12.7°. By setting in this way, the spread angles ofthe light after exiting from an optical fibers 42 a may be increased aswide as possible and the illumination may be homogenized at a shorterdistance from the exit end faces of the optical fibers 42 a. The beamexpander optical system 55 may be disposed between the homogenizer 40and the light condensing member 41, but the beam expander optical system55 is preferably disposed at a position immediately upstream of thehomogenizer 40 (immediately before the homogenizer 40), as shown in FIG.9, from the viewpoint of control.

The beam expander optical system 55 described above may be produced bycombining concave lenses and convex lenses, and the like, according tothe numerical apertures of the optical fibers 42 a.

As described above, also in the acoustic wave detection probe accordingto the present embodiment, light (measuring light) is passed through thehomogenizer once to flat-top the energy profile, and then the beamdiameter of the laser light when entering the bundle fiber is controlledby the light condensing member. This may provide identical effects tothose of the first embodiment.

In addition, the measuring light is expanded to an optimum beam diameterwith respect to the angular apertures of a plurality of optical fibers42 a in the bundle fiber 42 using the beam expander optical system 55 inthe present embodiment, so that the homogeneity of illumination mayfurther be improved.

[Third Embodiment of Acoustic Wave Detection Probe]

Next, a third embodiment of the acoustic wave detection probe will bedescribed. The probe of the present embodiment differs from the firstembodiment in that it projects light guided by the bundle fiber 42 via alight guide plate. Therefore, components identical to those of the firstembodiment will not be elaborated upon further here unless otherwisespecifically required.

FIG. 10 illustrates light guide plate configuration examples. FIG. 11 isa schematic view illustrating the arrangement of the acoustic wavetransducer, the optical fibers, and the light guide plate in the probeof the present embodiment, in which a is a cross-sectional view viewedfrom the front and b is a cross-sectional view viewed from the side.

The probe 11 according to the present embodiment includes a light guidesection which is formed of the homogenizer, the light condensing member,the fusion processed bundle fiber 42, and a light guide plate 43, theacoustic wave transducer array 20, and the housing 11 a that holds theexit end E2 of the bundle fiber 42 and the acoustic wave transducerarray 20. In the present embodiment, the probe 11 is used throughoptical connection to the laser unit such that laser light L outputtedfrom the laser unit enters the homogenizer. The laser light L incidenton the homogenizer enters the entrance end of the bundle fiber 42 viathe light condensing member. Thereafter, the laser light L guided by thebundle fiber 42 is directly enters a connection surface S1 of the lightguide plate from the exit ends E2 of a plurality of optical fibers 42 ain the bundle fiber 42, then the laser light L guided by the light guideplate 43 exits from an exit surface S2 of the light guide plate and isprojected onto a subject M as measuring light.

The housing, homogenizer, the light condensing member, and the acousticwave transducer array 20 are identical to those of the first embodiment.

<Light Guide Plate>

The light guide plate 43 is, for example, an acrylic plate or a silicaplate with a surface being specially processed to cause light enteredfrom one end (connection surface S1) to be uniformly surface-emittedfrom the other end (exit surface S2). For example, the light guide plate43 may be produced by forming thin resin films 43 b having a lowrefractive index on a pair of opposite side surfaces of a silica plate43 a, as shown in FIG. 10. In this case, laser light entered from theconnection surface S1 propagates while repeating reflections at theinterface S3 between the silica plate 43 a and the thin resin film 43 band exits from the exit surface S2. The exit ends E2 of the opticalfibers 42 a are disposed almost evenly on the connection surface S1 ofthe light guide plate 43 and optically connected thereto. As illustratedin a of FIG. 10, if the light guide plate 43 has a tapered shape thatextends toward the exit surface S2 from the connection surface S1 of thelight guide plate, it is possible to uniformly project the laser light Lmore extensively. Note that the light guide plate 43 may have a cuboidshape as shown in c of FIG. 10. As illustrated in FIG. 11, two lightguide plates 43 are disposed so as to face to each other across theacoustic wave transducer array 20, and optical fibers 42 a are connectedto the connection surface S1 of each light guide plate 43 in the presentembodiment. The light guide plate 43 may have, at a tip portion thereof,a mechanism that diffuses light (resin containing scattering particlesor the like) or a mechanism that directs the traveling direction oflight to the acoustic wave transducer array 20 (notch for refractinglight or the like), so that the laser light L can be projected onto awider range of the subject M.

As described above, also in the acoustic wave detection probe accordingto the present embodiment, light (measuring light) is passed through thehomogenizer once to flat-top the energy profile, and then the beamdiameter of the laser light when entering the bundle fiber is controlledby the light condensing member. This may provide identical effects tothose of the first embodiment.

In addition, the measuring light is projected via the light guide platein the present embodiment, so that the homogeneity of the energy profileof light projected on to the subject may further be improved.

<Design Changes of Probe>

In the present invention, the measuring light is passed through thehomogenizer once to flat-top the energy profile. But, there may be acase in which it is difficult to make the energy profile completelyflat-topped even in the case where the homogenizer is used.Consequently, it is preferable to consider the positions of opticalfibers in the bundle fiber when disposing the exit ends of the opticalfibers in the present invention.

For example, the energy profile of laser light is generally a Gaussiandistribution centered on the optical axis. In this case, there may be acase in which local intensity imbalance which is dependent on thedistance from the optical axis occurs even in the case where thehomogenizer is used. More specifically, for example, in the energyprofile shown in a of FIG. 6, there is a region where the intensity isgradually decreased from the flat-topped region to the periphery.Consequently, for example, a plurality of optical fibers is divided intoa divided area close to the center of the bundle fiber 42 (center sidearea 62) and a divided area close to the periphery (peripheral side area64) in the end face arrangement of the entrance end as shown in FIG. 12.Then, optical fibers 62 a belonging to the center side area 62 andoptical fibers 64 a belonging to the peripheral side area 64 areuniformly disposed according to the relative magnitude with respect toeach divided area. The term “disposed” here refers to include that theexit ends of the optical fibers 62 a and 64 a are evenly disposed aroundthe acoustic wave transducer array 20, as shown, for example, in a ofFIG. 13 and that the exit ends of the optical fibers 62 a and 64 a areevenly disposed on the connection surface of the light guide plate 43,as shown for example in b of FIG. 13. The term “uniformly according tothe relative magnitude with respect to each divided area” to which theexit ends belongs as used herein refers to that the ratio of the numbersof the optical fibers of each divided area is not necessarily 1 to 1,and that the exit ends of optical fibers belonging to each divided areaare disposed in a mixed manner as a whole according to the ratio of thenumber of optical fibers belonging to each divided area. Thisarrangement allows the local intensity imbalance described above to beinhibited and the homogeneity of the energy profile of the measuringlight actually projected onto the subject to be improved.

The area division method is not limited to the above describe methodand, for example, the bundle fiber may be divided into three areasaccording to the distance of the center thereof or divided into sixequal areas with respect to the angle around the center thereof.

Further, in the case where the components of the light guide section areintegrally held by the holding section, the holding section preferablyhas a connector structure removably attachable to a mounting section ofthe equipment housing which includes a light source. For example, a ofFIG. 14 is a schematic view illustrating the structure of a mountingsection 69 of an equipment housing 68 which includes a laser unit 13(light source) and a holding section 65 a. The equipment housing 68includes therein the laser unit 13, and the laser unit 13 and a lightshaping diffuser 53 (homogenizer) are optically connected when theholding section 65 a is mounted in the mounting section 69.

The connector structure of a holding section 65 a is basicallyidentical, for example, to that of the holding section 60 a shown in aof FIG. 4 but differs in that it has a projection section 66 movable inup-down directions on the plane of the drawing by a resilient member 67,such as a spring or the like. The projection section 66 is pressed downinto the slot of the holding section 65 a when an external force isapplied from above and returns back by the resilient power of theresilient member 67 when the external force is eliminated. Note that thesurface of the protruding portion of the projection section 66 forms acurved surface so that the projection section 66 is also pressed downinto the slot of the holding section 65 a when an external force isapplied from a horizontal direction on the plane of the drawing. In themeantime, the mounting section 69 is provided with an engaging section69 a which is, for example, a slot having a complementary shape with theprojection section 66, as shown in a of FIG. 14. When insertion of theholding section 65 a into the mounting section 69 is started, theprojection section 66 is pressed down by the inner wall of the mountingsection 69 and, thereafter, when the projection section 66 reaches theengaging section 69 a, the projection section 66 returns back andengages with the engaging section 69 a. Then, outputted laser light L isguided to the light shaping diffuser 53 by an optical system 70 and,thereafter, will propagate through the probe of the present invention.

As an alternative example in which the holding section has a connectorstructure removably attachable to the mounting section, an embodimentshown in FIG. 15 may be cited.

The connector structure of a holding section 65 b is basicallyidentical, for example, to that of the holding section 60 b shown in bof FIG. 4 but differs in that it has a projection section 66 movable inup-down directions on the plane of the drawing by a resilient member 67,such as a spring or the like. The projection section 66 is identical tothe foregoing projection section. In the meantime, the mounting section69 is provided with an engaging section 69 a which is, for example, aslot having a complementary shape with the projection section 66 and alight shaping diffuser 53, as illustrated in a of FIG. 15. Wheninsertion of the holding section 65 b into the mounting section 69 isstarted, the projection section 66 is pressed down by the inner wall ofthe mounting section 69 and, thereafter, when the projection section 66reaches the engaging section 69 a, the projection section 66 returnsback and engages with the engaging section 69 a. At the same time, thepositions of the light shaping diffuser 53 and the light condensingmember 41 are fixed and can be optically connected. Then, outputtedlaser light L is guided to the light shaping diffuser 53 by an opticalsystem 70 and, thereafter, will propagate through the probe of thepresent invention. This embodiment is preferable because only diffusedmeasuring light is outputted from the equipment even when the probe isnot connected.

As still another example in which the holding section has a connectorstructure removably attachable to the mounting section, an embodimentshown in FIG. 16 may be cited.

For example, a holding section 65 c holds the entrance end so as tocover the entrance face of the bundle fiber 42, and has a projectionsection 66 identical to the foregoing projection section and a windowsection 74 at a portion where the laser light L enters. The windowsection 74 is formed of an optically transparent material (e.g., silica)and provided on the optical path of the laser light L so as to block upa groove accommodating the entrance face of the bundle fiber 42. Thiscauses, for example, the entrance face of the bundle fiber 42 to beplaced in a space sealed by the holding section 65 c. The surface of thewindow section 74 on the light source side preferably has ananti-reflection coat (AR coat), such as MgF₂ film, Ta₂O₅ film, or SiO₂multilayer film. In the meantime, the mounting section 69 is providedwith, for example, a beam expander 73, a homogenizer 40, and a lightcondensing member 41, other than an engaging section 69 a which is aslot having a complementary shape with the projection section 66, asshown in FIG. 16. The beam expander 73 is formed of, for example, aplanar-concave lens 71 and a convex lens 72. The mounting procedure ofthe holding section 65 c in the mounting section 69 is identical to thatdescribed above. When the holding section 65 c is mounted in themounting section 69, laser light L passed through the beam expander 73,homogenizer 40, and light condensing member 41 enters the entrance faceof the bundle fiber 42 by transmitting through the window section 74. Asadherence of dust and the like occurs on the light source side surfaceof the window section 74 having a low energy density in comparison withthe entrance face of the bundle fiber 42, this embodiment has anadvantageous effect that end face damage is unlikely to occur. Also inFIG. 14 or 15, identical effect may be obtained if a window section isprovided. As the light condensing member 41 is installed on the lightsource system side, the embodiment also proves an advantageous effectthat, when the holding section 65 c is attached to the mounting section65, the angular accuracy requirement for the holding section 65 c isrelaxed and only the positional accuracy needs to be primarilyconsidered.

Further, an ND filter may be used as the window section 74 in FIG. 16.For example, the ND filter is a silica substrate coated with amultilayer oxide film. In such a case, the intensity of the laser lightL may be reduced on the probe side and an adjustment mechanism for laserlight intensity is unnecessary on the light source system side.

As shown, for example, in FIG. 17, the holding section preferably has anaperture member having an aperture that allows the measuring light,which is to enter the bundle fiber, to pass through and a taperstructure at the entrance end of the bundle fiber 42. The aperturediameter is formed so as to be gradually reduced toward the entrance endto a size corresponding to the diameter of the bundle fiber. Forexample, the aperture diameter of the aperture member 75 on the bundlefiber side corresponds to the diameter of the bundle fiber in FIG. 17.The taper angle of the taper structure is preferably larger than theconvergence angle of light entering the bundle fiber 42 and smaller thanthe NA of the optical fiber. The inner surface of the aperture member 75is formed to reflect, scatter, or absorb light. If the aperture innersurface reflects or scatters light, an angular light component outsidethe incident angle range within which the optical fibers can receivelight becomes able to enter the optical fibers by way of the reflectionor the scattering, so that the light transmission efficiency is furtherimproved. In the meantime, if the aperture inner surface absorbs lightthe light of angular component outside the incident angle which can bereceived by the optical fibers is absorbed at a location away from theoptical fibers and by a wide area, so the damage to the optical fibersis more inhibited than in the case where the absorption takes placeadjacent to the optical fibers.

In order to cause the aperture inner surface to reflect light, forexample, it is only necessary to perform smoothing processing, such asmirror finish and the like, or to form a high reflective film, such as agold thin film or the like, on the aperture inner surface. Further, inorder to cause the aperture inner surface to scatter light, it is onlynecessary to form the aperture member with, for example, a pressurizedpowder body or a sintered body of ceramics, such as Al₂O₃, TiO₂, ZrO₂,or the like, or with Teflon® or unpolished glass. In order to cause theaperture inner surface to absorb light, it is only necessary to form theaperture member with, for example, a metal such as aluminum, brass,copper, or the like.

In the case where the ambient damage mode is likely to occur due to theinfluence of the peripheral portion of the energy profile, it ispreferably to provide a light guide member for preventing damage tosurrounding members of the bundle fiber (ferrule and the like) as shown,for example, in FIG. 18A, 18B, or 20. For example, the light guidemember shown in FIG. 18A or 18B is formed of a cap member and a ringshaped chip 77 made of a material resistant to optical energy (e.g.,high light absorption sapphire in the wavelength range of measuringlight used) and fitted around the cap member. As for the cap member, forexample, a silica rod 76 a (FIG. 18A) or an air gap optical fiber 76 b(FIG. 18B) may be used. The chip 77 is fitted around the entrance end ofthe cap member. If the air gap optical fiber 76 b is used, inparticular, it is preferable that the chip 77 is embedded in theconnector of the air gap optical fiber 76 b. For example, the air gapoptical fiber 76 b shown in FIG. 18B is provided with an exit sideconnector 65 e removably attachable to a connector 65 d of the bundlefiber 42 and an entrance side connector 65 f, and the chip 77 isembedded at least in the entrance side connector 65 f. The use of such alight guide member allows the light in the peripheral portion to beblocked, for example, through absorption or reflection by the chip 77(FIG. 19). Consequently, the light at the peripheral portion isprevented from reaching the surrounding members of the bundle fiber andthe ambient damage mode is prevented from occurring.

In the meantime, the light guide member shown in, for example, FIG. 20is formed of a first aperture stop 78 (for blocking peripheral portion),a second aperture stop 79 (for adjusting light intensity), a relay lenssystem 80, and a third aperture stop 81 (for blocking peripheralportion). For example, the first aperture stop 78 is disposed adjacentto the focal point of the light condensing member 41, the secondaperture stop 79 is disposed adjacent to the relay lens system 80, andthe third aperture stop 81 is disposed adjacent to the entrance end ofthe bundle fiber. The use of such a light guide member allows the beamdiameter to be enlarged or reduced before and after the relay lenssystem 80, and if the diameter of the bundle fiber differs depending onthe probe used, the beam diameter may be adjusted to a desired size byadjusting the relay lens system 80. The peripheral portion of light isblocked by the first aperture stop 78 and the third aperture stop 81,while the second aperture stop 79 is used for light intensityadjustment.

Further, for example, the light guide member formed of the silica rod 76a and chip 77 (FIG. 18A), and the light guide member formed of theaperture stops 78, 79, 81 and the relay lens system 80 (FIG. 20) mayalso be provided in the holding member shown in FIG. 4, 14, or 15.

The provision of the connector structure in the holding section which isremovably attachable to the mounting section as described above mayimprove convenience as a probe. Note that the connector structure is notlimited to those described above and it is preferable that the holdingsection is compact.

[First Embodiment of Photoacoustic Measurement Apparatus]

Next, a first embodiment of photoacoustic measurement apparatus will bedescribed. In the present embodiment, a detailed description will bemade of a case in which the photoacoustic measurement apparatus is aphotoacoustic image generation apparatus that generates a photoacousticimage based on photoacoustic signals. FIG. 21 is a block diagram of thephotoacoustic image generation apparatus 10 of the present embodiment,illustrating the configuration thereof.

The photoacoustic image generation apparatus 10 of the presentembodiment includes a probe 11 according to the present invention, anultrasonic unit 12, a laser unit 13, an image display means 14, and aninput means 16.

<Laser Unit>

The laser unit 13 corresponds to the light source of the presentinvention and outputs, for example, laser light L as measuring light tobe projected onto a subject M. The laser unit 13 is configured to outputthe laser light L, for example, by receiving a trigger signal from acontrol means 29. The laser light L outputted from the laser unit 13 isguided to the probe 11 using a light guide means, such as optical fiber,and projected onto the subject M from the probe 11. Preferably, thelaser unit 13 outputs pulsed light with a pulse width of 1 to 100 nsecas the laser light.

Preferably, the pulse width tp (ns) of the laser light L satisfiesFormula 5 below. Here, A is the pulse energy (J) of the laser light usedwhen entering the bundle fiber, λ is the wavelength (nm) of the laserlight used, G is the damage threshold energy density (J/mm²) of thebundle fiber, λG and tG are respectively the wavelength and the pulsewidth of the laser light for which the damage threshold energy densityis obtained, and d is the diameter of the bundle fiber. The reason forthe above is that Formula 6 below is preferably satisfied in order toprevent the end face damage of the bundle fiber.

$\begin{matrix}{{\left( {\frac{4\;{Ad}^{2}}{\pi\; G}\frac{\lambda_{G}}{\lambda}} \right)^{2}t_{G}} < t_{P}} & {{Formula}\mspace{14mu} 5} \\{A < {{\pi\left( \frac{d}{2} \right)}^{2}{G\left( \frac{\lambda}{\lambda_{G}} \right)}\sqrt{\frac{t_{P}}{t_{G}}}}} & {{Formula}\mspace{14mu} 6}\end{matrix}$

For example, the laser unit 13 is a Q-switch (Qsw) alexandrite laser inthe present embodiment. In this case, the pulse width of the laser lightL is controlled, for example, by the Qsw. The wavelength of the laserlight is determined appropriately based on the light absorptioncharacteristics of the measurement target substance within the subject.If the measurement target is hemoglobin in a living body (i.e., if bloodvessel is imaged) the wavelength, in general, is preferably belongs tonear infrared wavelength range. The near infrared wavelength rangerefers to the wavelength range of about 700 to 800 nm. The laser light Lmay have a single wavelength or a plurality of wavelengths (e.g., 750 nmand 800 nm). If the laser light has a plurality of wavelengths, thesewavelengths of light may be projected onto the subject M simultaneouslyor alternately by switching.

<Probe>

The probe 11 is a probe of the present invention that detectsphotoacoustic wave U generated in the subject M and it is the probeaccording to the third embodiment in the present embodiment.

<Ultrasonic Unit>

The ultrasonic unit 12 includes a receiving circuit 21, an AD conversionmeans 22, a receive memory 23, a photoacoustic image reconstructionmeans 24, a detection•log conversion means 27, a photoacoustic imageconstruction means 28, the control means 29, an image combining means38, and an observation method selection means 39. For example, thereceiving circuit 21, AD conversion means 22, receive memory 23,photoacoustic image reconstruction means 24, detection•log conversionmeans 27, and photoacoustic image construction means 28 correspond, as aunit, to the acoustic image generation means of the present invention.

The control means 29 controls each section of the photoacoustic imagegeneration apparatus 10 and includes a trigger control circuit 30 in thepresent embodiment. The trigger control circuit 30 sends a light triggersignal to the laser unit 13 when, for example, activating thephotoacoustic image generation apparatus. This causes a flash lamp inthe laser unit 13 to be turned on and excitation of the laser rod isstarted. The excitation state of the laser rod is maintained and thelaser unit 13 becomes ready to output pulsed laser light.

Then, the control means 29 subsequently sends a Qsw trigger signal tothe laser unit 13 from the trigger control circuit 30. That is, thecontrol means 29 controls the output timing of the pulsed laser lightfrom the laser unit 13 by the Qsw trigger signal. Further, the controlmeans 29 sends a sampling trigger signal to the AD conversion means 22simultaneously with the transmission of the Qsw trigger signal in thepresent embodiment. The sampling trigger signal serves as a timingsignal to start sampling of the photoacoustic signal in the ADconversion means 22. In this way, the use of the sampling trigger signalallows the photoacoustic signal to be sampled in synchronization withthe output of the pulsed laser light.

The receiving circuit 21 receives a photoacoustic signal detected by theprobe 11. The photoacoustic signal received by the receiving circuit 21is sent to the AD conversion means 22.

The AD conversion means 22 is a sampling means, and samples thephotoacoustic signal received by the receiving circuit 21 and convertsit to a digital signal. For example, the AD conversion means 22 includesa sampling control section and an AD converter. The receive signalreceived by the receiving circuit 21 is converted to a digitized sampledsignal by the AD converter. The AD converter is controlled by thesampling control section and configured to perform sampling when asampling trigger signal is received by the sampling control section. TheAD conversion means 22 samples the receive signal at a predeterminedsampling period based on, for example, an AD clock signal ofpredetermined frequency inputted from outside.

The receive memory 23 stores the photoacoustic signal sampled by the ADconversion means 22 (i.e., the sampled signal described above). Then,the receive memory 23 outputs the photoacoustic signal detected by theprobe 11 to the photoacoustic image reconstruction means 24.

The photoacoustic image reconstruction means 24 reads the photoacousticsignal from the receive memory 23 and generates data of each line of aphotoacoustic image based on the photoacoustic signal detected by theacoustic wave transducer array 20 of the probe 11. The photoacousticimage reconstruction means 24 generates data of one line by adding up,for example, data from 64 acoustic wave transducers of the probe 11 atdelay times corresponding to the positions of the acoustic wavetransducers (delay-and-sum method). The photoacoustic imagereconstruction means 24 may perform the reconstruction by the CBP(Circular Back Projection) method in place of the delay-and-sum method.Otherwise, the photoacoustic image reconstruction means 24 may performthe reconstruction by the Hough transform method or the Fouriertransform method.

The detection•log conversion means 27 obtains an envelope of the data ofeach line and performs log conversion on the obtained envelope.

The photoacoustic image construction means 28 constructs a photoacousticimage of one frame based on the log-converted data of each line. Thephotoacoustic image construction means 28 constructs a photoacousticimage, for example, by converting the position of the photoacousticsignal (peak portion) in the time axis direction to the position in thedepth direction of the photoacoustic image.

The observation method selection means 39 selects a display mode of thephotoacoustic image. As for the display mode of the volume data ofphotoacoustic signal, for example, a three-dimensional image displaymode, a tomographic image display mode, and a graphic display mode on apredetermined axis may be cited. Which display mode is to be used forthe display is determined by initial setting or selected according tothe user input via the input means 16.

The image combining means 38 generates volume data using thesequentially obtained photoacoustic signals. The generation of thevolume data is performed by allocating the signal value of eachphotoacoustic signal in a virtual space according to the coordinatesrelated to each photoacoustic image frame and pixel coordinates in thephotoacoustic image. For example, the coordinate when the Qsw triggersignal is sent, the coordinate when the light is actually outputted, thecoordinate when the sampling of the photoacoustic signal is started, andthe like are related to each photoacoustic image frame. In allocatingsignal values, if positions where signal values are to be allocatedoverlap, for example, an average value or a maximum value of the signalsis used as the signal value of the overlapped positions. Further, if nosignal value to be allocated is present, an interpolation is preferablyperformed, as required, using signal values of adjacent positions. Forexample, the interpolation is performed by allocating a weighted averageof four proximal points in order from the most proximal point to theinterpolating position. This allows more natural form of volume data tobe generated. The image combining means 38 further performs necessaryprocessing (e.g., scale correction, coloring according to the voxelvalue, and the like) on the generated volume data.

Further, the image combining means 38 generates a photoacoustic imageaccording to the observation method selected by the observation methodselection means 39. The photoacoustic image generated according to theselected observation method is the final image to be displayed on theimage display means 14 (display image). In the photoacoustic imagegeneration method described above, it should be appreciated that, aftera photoacoustic image is generated, the user may rotate or move theimage, as required. That is, in the case where a three-dimensional imageis displayed, if the user sequentially specifies or moves the directionof viewpoint using the input means 16, the photoacoustic image will berecalculated and the three-dimensional image will be rotated. The usermay also change the observation method, as appropriate, using the inputmeans 16.

The image display means 14 displays the display image generated by theimage combining means 38.

As described above, also in the photoacoustic measurement apparatusaccording to the present embodiment, light (measuring light) is passedthrough the homogenizer once to flat-top the energy profile, and thenthe beam diameter of the laser light when entering the bundle fiber iscontrolled by the light condensing member. As a result of this, it ispossible to transmit high energy light and eliminate imbalance in theamount of energy of light traveling through each of a plurality ofoptical fibers in photoacoustic measurements.

Then, as a result of this, stronger and homogenous photoacoustic signalsmay be obtained and high quality photoacoustic images may be generated.Further, the size and weight reduction of the transmission cable becomespossible, whereby the operability of the photoacoustic measurementapparatus is improved.

[Second Embodiment of Photoacoustic Measurement Apparatus]

A second embodiment of photoacoustic measurement apparatus will bedescribed next. Also in the present embodiment, a detailed descriptionwill be made of a case in which the photoacoustic measurement apparatusis a photoacoustic image generation apparatus. FIG. 22 is a blockdiagram of the photoacoustic image generation apparatus 10 of thepresent embodiment, illustrating the configuration thereof. The presentembodiment differs from the first embodiment in that it generates anultrasonic image in addition to the photoacoustic image. Therefore, thedetailed description of the components identical to those of the firstembodiment is omitted unless otherwise specifically required.

The photoacoustic image generation apparatus 10 of the presentembodiment includes a probe 11 according to the present invention, anultrasonic unit 12, a laser unit 13, an image display means 14, and aninput means 16, as in the first embodiment.

<Ultrasonic Unit>

The ultrasonic unit 12 of the present embodiment further includes atransmission control circuit 33, a data separation means 34, anultrasonic image reconstruction means 35, a detection•log conversionmeans 36, and an ultrasonic image construction means 37 in addition tothe configuration of the photoacoustic image generation apparatus shownin FIG. 21. In the present embodiment, the receiving circuit 21, ADconversion means 22, receive memory 23, data separation means 34,ultrasonic image reconstruction means 24, detection•log conversion means27, photoacoustic image construction means 28, ultrasonic imagereconstruction means 35, detection•log conversion means 36, andultrasonic image construction means 37 correspond, as a unit, to theacoustic image generation means of the present invention, as the signalprocessing means.

In the present embodiment, the probe 11 outputs (transmits) anultrasonic wave to a subject and detects (receives) a reflectedultrasonic wave of a transmitted ultrasonic wave from the subject, inaddition to the detection of a photoacoustic signal. As for the acousticwave transducer that performs transmission and reception of ultrasonicwaves, the acoustic wave transducer array 20 described above may be usedor a new acoustic wave transducer array for transmission and receptionof ultrasonic waves provided separately in the probe 11 may be used.Further, the transmission and reception of the ultrasonic wave may beseparated. For example, an ultrasonic wave may be transmitted from aposition different from the probe 11 and a reflected ultrasonic wave ofthe transmitted ultrasonic wave may be received by the probe 11.

The trigger control circuit 30 sends an ultrasonic wave transmissiontrigger signal that instructs transmission of an ultrasonic wave to thetransmission control circuit 33 when generating an ultrasonic image. Inresponse to the trigger signal, the transmission control circuit 33causes an ultrasonic wave to be transmitted from the probe 11. After thetransmission of the ultrasonic wave, the probe 11 detects a reflectedultrasonic wave from the subject.

The reflected ultrasonic wave detected by the probe 11 is inputted tothe AD conversion means 22 via the receiving circuit 21. The triggercontrol circuit 30 sends a sampling trigger signal to the AD conversionmeans 22 in conjunction with the transmission timing of the ultrasonicwave to cause the sampling of the reflected ultrasonic wave to bestarted. Here, whereas the reflected ultrasonic wave reciprocatesbetween the probe 11 and the ultrasonic wave reflection point, thephotoacoustic signal travels one way from the point of generation to theprobe 11. As the detection of reflected ultrasonic wave takes twice aslong as the detection of a photoacoustic signal generated at the samedepth, the sampling clock of the AD conversion means 22 may be reducedto half that of the photoacoustic signal sampling, for example, 20 MHz.The AD conversion means 22 stored a sampled signal of reflectedultrasonic wave in the receive memory 23. Either the sampling ofphotoacoustic signal or the sampling of reflected ultrasonic wave mayprecede the other.

The data separation means 34 separates the sampled signal ofphotoacoustic image from the sampled signal of reflected ultrasonic wavestored in the receive memory 23. The data separation means 34 inputs theseparated sampled signal of photoacoustic image to the photoacousticimage reconstruction means 24. The generation of a photoacoustic imageis performed in the same manner as in the first embodiment. In themeantime, the data separation means 34 inputs the separated sampledsignal of reflected ultrasonic wave to the ultrasonic imagereconstruction means 35.

The ultrasonic image reconstruction means 35 generates data of each lineof an ultrasonic image based on the reflected ultrasonic waves (sampledsignals thereof) detected by a plurality of acoustic wave transducers ofthe probe 11. For the generation of data of each line, the delay-and-summethod and the like may be used as in the generation of data of eachline in the photoacoustic image reconstruction means 24. Thedetection•log conversion means 36 obtains an envelope of the data ofeach line outputted from the ultrasonic image reconstruction means 35and performs log conversion on the obtained envelope.

The ultrasonic image construction means 37 generates an ultrasonic imagebased on the log-converted data of each line.

The image combining means 38 combines the photoacoustic image and theultrasonic image. For example, the image combining means 38 combines thephotoacoustic image and the ultrasonic image by superimposition. Thecombined image is displayed on the image display means 14. It is alsopossible to display the photoacoustic image and the ultrasonic image onthe image display means side-by-side or by switching, without performingthe image combining.

As described above, also in the photoacoustic measurement apparatusaccording to the present embodiment, light (measuring light) is passedthrough the homogenizer once to flat-top the energy profile, and thenthe beam diameter of the laser light when entering the bundle fiber iscontrolled by the light condensing member. This may provide advantageouseffects identical to those of the first embodiment.

Further, the photoacoustic measurement apparatus of the presentembodiment generates an ultrasonic image, in addition to a photoacousticimage. Therefore, a portion which cannot be imaged by the photoacousticimage may be observed by referring to the ultrasonic image.

In the foregoing, the description has been made of a case in which thephotoacoustic measurement apparatus generates a photoacoustic image oran ultrasonic image. But the image generation is not necessarilyrequired. For example, photoacoustic measurement apparatus may also beconfigured to measure the presence/absence or physical quantity of ameasuring target based on the magnitude of photoacoustic signal.

What is claimed is:
 1. An acoustic wave detection probe provided with a light guide section, and an acoustic wave transducer, wherein the light guide section comprises: a homogenizer that flat-tops an energy profile of high energy measuring light entered the light guide section; a light condensing member that condenses the high energy measuring light transmitted through the homogenizer; and a bundle fiber which includes a plurality of optical fibers and is disposed such that the high energy measuring light transmitted through the light condensing member enters from an entrance end of the bundle fiber, wherein the acoustic wave detection probe comprises a holding section that integrally holds the light condensing member and the bundle fiber; the homogenizer further diffuses the high energy measuring light; when the focal length of the light condensing member, the spread angle of the high energy measuring light when entering the homogenizer, and the diffusion angle of the homogenizer are taken as f, φ, and θ respectively, the light condensing member condenses the high energy measuring light such that a minimum beam diameter D defined by Formula 1 below satisfies Formula 2 below in relation to a diameter d of the bundle fiber; and the bundle fiber is disposed such that the high energy measuring light enters the bundle fiber with the beam diameter D being 0.8 d to 1.2 d; $\begin{matrix} {D = {2.5 \cdot f \cdot {\tan\left( \sqrt{\left( \frac{\phi}{2} \right)^{2} + \left( \frac{\theta}{2} \right)^{2}} \right)}}} & {{Formula}\mspace{14mu} 1} \\ {{0.8\mspace{14mu} d} \leqq D \leqq {1.2\mspace{14mu} d}} & {{Formula}\mspace{14mu} 2} \end{matrix}$ the light guide section comprises a beam expander optical system immediately before the entrance side of the homogenizer, the beam expander optical system having an expansion factor that conforms to angular apertures of the plurality of optical fibers so that the high energy measuring light is expanded to a beam diameter that conforms to the angular apertures of the plurality of optical fibers; the homogenizer is a light shaping diffuser in which small lenses are disposed randomly on one side of a substrate; the light guide section guides the high energy measuring light such that the high energy measuring light is outputted toward a subject and the acoustic wave transducer detects a photoacoustic wave generated in the subject by the projection of the high energy measuring light; and the minimum beam diameter D is set to inhibit damage to the bundle fiber and eliminate the imbalance in the amount of energy between each light traveling through each optical fiber of the bundle fiber.
 2. The probe as claimed in claim 1, wherein the holding section integrally holds the homogenizer as well.
 3. The probe as claimed in claim 1, wherein the acoustic wave detection probe comprises an aperture member having an aperture that allows the measuring light, which is to enter the bundle fiber, to pass through and an aperture member which is provided at the entrance end of the bundle fiber and gradually reduces the diameter of the aperture toward the entrance end to a size corresponding to the diameter of the bundle fiber.
 4. The probe as claimed in claim 1, wherein the holding section includes therein a light guide member formed of a cap member and a ring shaped chip made of a material resistant to optical energy and fitted around the cap member.
 5. The probe as claimed in claim 1, wherein the holding section includes therein a light guide member having an aperture stop and a relay lens system.
 6. The probe as claimed in claim 1, wherein the holding section comprises a connector structure removably attachable to a mounting section of an equipment housing which includes a light source.
 7. The probe as claimed in claim 1, wherein the entrance end of the bundle fiber is fusion processed.
 8. The probe as claimed in claim 1, wherein outer circumferences of the plurality of optical fibers are covered with a material having high durability against optical energy at the entrance end.
 9. The probe as claimed in claim 8, wherein the material having high durability against optical energy is silica.
 10. The probe as claimed in claim 1, wherein the light guide section comprises at least one light guide plate having a connection surface to which at least some of exit ends of the plurality of optical fibers are connected and an exit surface from which the measuring light entered from the connection surface exits.
 11. The probe as claimed in claim 10, wherein the light guide plate is provided in plurality and disposed across the acoustic wave transducer.
 12. A photoacoustic measurement apparatus, comprising: the probe of claim 1; and a signal processor that processes a photoacoustic signal of a photoacoustic wave detected by the acoustic wave transducer.
 13. The photoacoustic measurement apparatus as claimed in claim 12, wherein the signal processor comprises an acoustic image generator that generates a photoacoustic image based on the photoacoustic signal.
 14. The photoacoustic measurement apparatus as claimed in claim 13, wherein: the acoustic wave transducer detects a reflected acoustic wave of a transmitted acoustic wave to the subject; and the acoustic image generator generates a reflected acoustic wave image based on a signal of the reflected acoustic wave. 